1. Field of the Invention
This invention relates to an objective lens driving apparatus, and more particularly to an objective lens driving apparatus for an optical reproduction system.
2. Description of the Prior Art
In an optical reproduction system using laser light, signals of information are detected through an objective lens that focuses beams of the laser light. However, in order to accurately detect such signals, a focusing control and a tracking control are necessary. The focusing control automatically focuses the light beam on an information recording medium despite unevenness and vibrations of the medium. The tracking control causes the optical spot of the focused light beam to automatically track a signal track on the medium. To perform the focusing and tracking controls, an error detecting apparatus, which detects errors caused by unevenness and vibration of the medium, is necessary. Also, an objective lens driving apparatus, which drives the objective lens so as to cancel such errors, is needed.
Conventional objective lens driving apparatus may be classified into two types in terms of operating principles. In one type, as shown in FIGS. 13 through 15, a movable unit supporting an objective lens rotates about an inertial main axis, and moves in a direction parallel to the inertial main axis. Thus, movements in two orthogonal directions, such as the tracking and focusing directions of the objective lens, can be achieved. In the other type, as shown in FIGS. 16 and 17, a movable unit supporting an objective lens moves directly in two orthogonal directions, such as the tracking and focusing directions of the objective lens.
The specific configurations and operating principles of the above-described types of objective lens driving apparatus will be described. FIG. 13 is a perspective view, FIG. 14 a sectional view taken along line A--A of FIG. 13, and FIG. 15 a sectional view taken along line B--B of FIG. 13 of the first type of prior art driving apparatus. FIG. 16 a perspective view, and FIG. 17 an exploded perspective view of the second type of prior art driving apparatus.
The objective lens driving apparatus shown in FIGS. 13 through 15 is configured as follows. In FIG. 13, a shaft 102 is implanted perpendicularly at the center of upper face of a base 101 made of a magnetic material. The shaft 102 is fitted into a sleeve bearing 103, which serves as a sliding bearing. A supporting sleeve 104 having a bottom wall 104a is rigidly secured to the sleeve bearing 103. Thus, the supporting sleeve 104 can slide in the axial direction of the shaft 102, and also can rotate about the shaft 102. An objective lens 105 is supported by the bottom wall 104a of the supporting sleeve 104. A sleeve portion 104b of the supporting sleeve 104 serves as a coil bobbin. A focusing coil 106 is used for controlling positions of the supporting sleeve 104 in the axial direction of the shaft 102. A tracking coil 107 is used for controlling the position of the supporting sleeve 104 in the circumferential direction of the shaft 102. The focusing and tracking coils 106 and 107 are rigidly secured to an outer surface of the sleeve portion 104b.
Inner yokes 108a and 108b project from the base 101 in symmetry with respect to the shaft 102. The yokes 108a and 108b are respectively opposed to an inner wall 104c of the sleeve portion 104b and also to an inner face 104d of the bottom wall 104a in a non-contact relation with each other. A permanent magnet 110 is disposed between outer yokes 109a and 109b and the base 101, yokes 109a and 109b are magnetized to form a magnetic field in the axial direction of the shaft 102. The outer yokes 109a and 109b are disposed outside the sleeve portion 104b. The outer yokes 109a and 109b are respectively opposed to the outer faces 108c and 108d of the inner yokes 108a and 108b in a non-contact relation with each other. A support 111 is disposed on the base 101 at a position inside the sleeve portion 104b of the supporting sleeve 104. A damper member 112 for setting a neutral position is provided between the support 111 and the sleeve bearing 103. The damper member 112 is made of a resilient material, such as rubber. In FIG. 14, a light-penetrating aperture 113 is provided at a position on the base 101, and directs beams of light to and from the objective lens 105.
In the above-described configuration, when the focusing coil 106 is energized, the position of the supporting sleeve 104 is changed by an electromagnetic force in the Y-axis direction, as shown in FIG. 13. Thus, when the exciting current is appropriately controlled, the focusing control can be performed. Further, when the tracking coils 107 are energized, the supporting sleeve 104 is slightly rotated by an electromagnetic force in the X direction, as shown in FIG. 13. Thus, when the exciting currents are appropriately controlled, the tracking control can be performed. The control of exciting currents is performed by means of a conventional servo control system (not shown).
FIGS. 16 and 17 show another example of a conventional objective lens driving apparatus. In FIG. 16, a metal rod-fixing plate 212 is attached to extend perpendicular to one end of a base 211 made of a magnetic material. One end of each of four metal rods 213, which are parallel to each other and also parallel to the base 211, is rigidly secured to metal-rod fixing plate 212. A movable unit 215 supporting an objective lens 214 is rigidly secured on the other ends of metal rods 213. A focusing coil 216 and tracking coils 217 are rigidly fixed to the movable unit 215. A pair of inner yokes 218 project from the base 211 so as to fit with a predetermined clearance, into openings 216a in the focusing coil 216. Further, a pair of outer yokes 219 project from the base 211 so as to sandwich the focusing coil 216 and the tracking coils 217 on opposite sides of the inner yokes 218. Two permanent magnets 220 are respectively fixed to the inner faces 219a of the outer yokes 219 on opposite sides of the inner yokes 218.
In the above-described configuration, when the focusing coil 216 is energized, the movable unit 215 is moved by an electromagnetic force in the Y direction, as shown in FIG. 17. Thus, focusing control can be performed. When the tracking coils 217 are energized, the movable unit 215 is moved by an electromagnetic force in the X direction, as shown in FIG. 16. Thus, tracking control can be performed.
However, both of the above-mentioned conventional apparatus have problems, which will be described hereinafter. In the apparatus shown in FIGS. 13 through 16, the damper member 112 serves to set a neutral position for the supporting sleeve 104. The damper member 112 is rigidly secured at a position opposite to the objective lens 105 with respect to the shaft 102, i.e., at a biased position. Thus, when a displacement is given in the focusing direction, a moment of rotation about an orthogonal axis of the supporting sleeve 104 is produced. As a result, a reaction force proportional to this moment is produced between the shaft 102 and the sleeve bearing 103. The sliding friction in the sleeve bearing 103 is substantially proportional to the vertical drag. Thus, the greater the displacement in the focusing direction, the larger the frictional force which is produced. Therefore, the relationship between displacement and forces exhibits a hysteresis loop, as shown in FIG. 18.
The apparatus shown in FIGS. 13 through 15 exhibits a large amount of hysteresis. This hysteresis causes problems in the accurate control of focusing and tracking. To reduce such problems, the inner surface of the sleeve bearing 103 and the surface of the shaft 102 may be finished with higher precision. However, such precision machining increases man-hours and production cost.
In the apparatus shown in FIGS. 16 and 17, when the objective lens 214 is moved in the tracking direction X, the point of application of force generated by the tracking coils 217 coincides with the center of gravity of the movable unit 215. As a result, the movable unit 215 is moved with the rods 213 in parallel. However, when the movable unit 215 is shifted from the neutral position in the focusing direction, the point of application of force generated by the tracking coils 217 deviates from the center of gravity of the movable unit 215. Thus, a moment force about the Z axis, as shown in FIG. 16, is produced. Consequently, the objective lens 215 becomes tilted, resulting in an increase of jitter.
If the movable unit was supported by a combination of parallel springs or a combination of parallel springs and bearings in place of the four metal rods 213, the force constraining the rotational displacement about the axis Z would become stronger. However, when the movable unit was shifted from a neutral position in the focusing direction, the movable unit still would be rotated about the axis Z. Thus, only at lower frequencies would the amount of tilting of the objective lens be reduced. However, at frequencies of about 1 kHz, tilting oscillation would occur, resulting in unstable control.
As described above, in the conventional objective lens driving apparatus, when displacement is made in the focusing direction, a rotational moment is inevitably produced. Thus, a large amount of hysteresis occurs. Also where the movable unit is shifted in a focusing or tracking direction, the balance of forces is destroyed, causing the objective lens to be tilted.